The present invention relates to a multibeam optical system that divides a laser beam emitted from a laser source into a plurality of beams and directs the beams toward an object surface such as a photosensitive medium. Particularly, the invention relates to the optical system that employs a diffractive beam-dividing element to divide a laser beam emitted from a multiline laser source.
The multibeam optical system needs a beam-dividing element that divides a laser beam emitted from a laser source into a plurality of beams to form a plurality of beam spots on the object surface.
A conventional multibeam optical system has.employed a prism-type beam splitter as the beam-dividing element, which comprises a plurality of prism blocks cemented to one another. The cemented faces of the prism blocks are provided with multi-layer coatings having the desired reflecting properties, respectively.
However, when employing a prism-type beam splitter, when each one of the multi-layer coatings can divide an incident beam only into two separate beams, the number of prism blocks corresponding to the required number of separate beams must be cemented to one another, and when cementing one block to another block, a positional error between two blocks unavoidably arises. Accordingly, when a large number of separate beams are required, the deviations of the beam spots on the object surface tend to become large due to an accumulation of positional errors between the cemented prism blocks.
It is therefore an object of the present invention to provide an improved multibeam optical system capable of reducing the deviations of the beam spots caused by the positional error of the prism blocks.
For the above object, according to the present invention, a diffractive beam-dividing element is used in place of a prism-type beam splitter. The diffractive beam-dividing element has a diffraction grating formed on a transparent plate and divides the incident laser beam by means of diffraction. Since the diffractive beam-dividing element is made of a single block that is not cemented, it does not generate any positional error even when the large number of the separate beams are required
The diffractive beam-dividing element diffracts the incident beam to be divided into a plurality of diffracted beams exiting at the different diffraction angles, respectively. The diffractive beam-dividing element is designed such that the light quantity of the incident beam is equally distributed among the plurality of diffracted beams at a predetermined design wavelength.
However, when employing the diffractive beam-dividing element, the distribution of the light quantity is affected by the wavelength of the incident beam and the light quantity of the incident beam cannot be equally distributed when the wavelength of the incident beam is different from the design wavelength, which causes an imbalance among the light quantities of the diffracted beams.
For example, when the optical system employs a multiline laser such as an argon laser that emits light at a plurality of peak wavelengths, the optical system loses the balance among the light quantities of the diffracted beams, because at least one peak wavelength of the incident beam is different from the design wavelength. Therefore, in order to equally distribute the light quantity of the incident beam by the diffractive beam-dividing element, it has been required to place a filter for passing a beam component of a selected peak wavelength before the diffractive beam-dividing element. Thus, the beam components of peak wavelengths other than the selected peak wavelength are cut off by the filter, which results in low energy efficiency.
The present invention further aims to provide an improved multibeam optical system capable of equally distributing the light quantity among the diffracted beams even when the multiline laser source and the diffractive beam-dividing element are employed, with high energy efficiency.
For the above object, according to the present invention, there is provided a multibeam optical system, which includes a multiline laser source that emits light at a plurality of peak wavelength and a diffractive beam-dividing element having a diffraction grating, satisfying the condition (1);
xcexS less than xcexM less than xcexL.xe2x80x83xe2x80x83(1)
where
xcexS the shortest wavelength among the plurality of peak wavelengths,
xcexL is the longest wavelength among the plurality of peak wavelengths, and
xcexM is a design wavelength of the diffraction grating at which the light quantity of the incident beam is equally distributed among the plurality of diffracted beams
The diffraction grating has a corrugated surface consisting of a plurality of periodical phase patterns arranged in parallel at a predetermined fixed pitch.
Since the wavelengths xcexS and xcexL are different from the design wavelength xcexM, the light quantity of the incident beam of the wavelengths xcexS and xcexL is unevenly distributed by the diffraction grating. In the specification, xe2x80x9crelative light quantityxe2x80x9d is a light quantity of the diffracted beam when the light quantity of the incident beam is defined as 1. Further the value subtracting relative light quantity of the diffracted beam at the design wavelength xcexM from the relative light quantity of a diffracted beam at a predetermined wavelength is referred to as a xe2x80x9cquantity deviationsxe2x80x9d at the predetermined wavelength.
When the condition (1) is satisfied, a quantity deviation at the wavelength xcexS has an inverse sign of a quantity deviation at the wavelength xcexL in every diffraction order. That is, when the quantity deviation at the wavelength xcexS in a predetermined diffraction order is larger than zero, the quantity deviation at the wavelength xcexL in the same diffraction order is smaller than zero. The converse is also true.
Therefore, the quantity deviations at the wavelengths xcexS and xcexL cancel each other out, which reduces the imbalance of the distribution among the light quantities of the plurality of diffracted beams.
Further, the diffraction grating in the multibeam optical system of the invention can be defined so as to satisfy the following conditions (2) and (3);
SDN less than SDSxe2x80x83xe2x80x83(2)
SDN less than SDLxe2x80x83xe2x80x83(3)
where
SDS is the standard deviation, i.e., the square root of the variance, of the light quantity of the diffracted beams by the diffractive beam-dividing element at the shortest peak wavelength xcexS among the plurality of peak wavelengths:
SDL is the standard deviation of the diffracted beams by the diffractive beam-dividing element at the longest peak wavelength xcexL among the plurality of peak wavelengths; and
SDN is the standard deviation of the diffracted beams by the diffractive beam-dividing element at the peak wavelength xcexN that satisfies xcexS less than xcexN less than xcexL.
When the conditions (2) and (3) are satisfied, a variation of the light quantities of the diffracted beams of the wavelength xcexN is smaller than variations of the light quantities of diffracted beams of the wavelengths xcexS and xcexL. In the other words, the wavelength xcexN is close to the design wavelength xcexM of the diffraction grating Therefore, the quantity deviations at the wavelengths xcexS and xcexL cancel each other out in the same manner as described above.
When the incident beam on the diffractive beam-dividing element consists of two wavelengths, which are the longest wavelength xcexL and the shortest wavelength xcexS, it is easier to balance the light quantities at the respective diffraction orders than the case where the incident beam consists of three or more wavelengths.
The diffractive beam-dividing element may divide the incident beam such that the quantities of the diffracted beams, which are the sums of the quantities of the plurality of peak wavelengths, become nearly equal to one another
Alternatively, the diffractive beam-dividing element may divide the incident beam such that exposure values on a photosensitive medium to be irradiated by the diffracted beams become nearly equal to one another when taking the spectral sensitivity of the photosensitive medium into account. The exposure value is the product of the spectral sensitivity of the photosensitive medium and the light quantity of the corresponding peak wavelength.